HMRC Compliance Check Letter 2026: What It Means and What to Do

Receiving a letter from HMRC about your tax affairs can be alarming, but the right response depends entirely on what type of letter it actually is. HMRC sends millions of compliance letters each year, ranging from informal nudges that require no response to serious formal investigations that carry significant legal consequences. Understanding which type you have received is the essential first step.

The Three Main Types of HMRC Compliance Letter

1. Nudge Letters (Informal Prompts)

Nudge letters -- sometimes called "one-to-many" letters -- are sent to large groups of taxpayers where HMRC's data analytics suggests a potential discrepancy. Common triggers in 2026 include:

• Offshore bank account data shared under Common Reporting Standard
• Property rental income not declared on self assessment returns
• Capital gains from share sales not reported
• Cryptoasset disposals identified through exchange data
• High-value lifestyle indicators not matching declared income

A nudge letter is not a formal enquiry. It does not open a legal investigation. You are not required to respond directly to HMRC. However, the letter is HMRC signalling that it has data and expects you to review your position.

What to do: Review the specific area mentioned. If you believe your returns are correct, document why and retain that documentation. If you find an error, make a voluntary disclosure -- this significantly reduces any penalty.

2. Formal Enquiry Under S9A TMA 1970

A formal enquiry is legally opened using Section 9A of the Taxes Management Act 1970. It gives HMRC statutory powers to request documents and information from you. Unlike a nudge letter, a formal enquiry:

• Must be opened within 12 months of the filing date of the return under enquiry (for on-time returns)
• Must be notified to you in writing
• Entitles you to ask HMRC to close the enquiry if they are taking too long (via a closure notice application to the First-tier Tribunal)

Formal enquiries can be:

• Aspect enquiries -- focused on one specific area (e.g., rental income, a particular expense claim)
• Full enquiries -- a comprehensive review of the entire return

HMRC does not have to tell you which type it is opening, though it usually becomes clear from the information request.

3. Code of Practice 9 (COP9)

COP9 is HMRC's most serious compliance procedure. It is used when HMRC has evidence of, or reason to suspect, deliberate tax fraud -- not just errors or careless behaviour.

A COP9 letter will explicitly say it is being issued under Code of Practice 9 and will offer the Contractual Disclosure Facility (CDF). The CDF works as follows:

• You have 60 days to either accept the CDF (and commit to full disclosure of all fraud) or reject it
• If you accept and make a complete disclosure, HMRC commits not to criminally prosecute you
• If you reject, or fail to make a complete disclosure, criminal prosecution remains possible

Do not respond to a COP9 letter without specialist legal or tax advice. The 60-day window is real, but what matters even more is ensuring your response is accurate, complete and properly structured. Getting it wrong can cost you far more than the original tax liability.

Your First 30 Days: A Practical Checklist

Whether the letter is a nudge or a formal enquiry, the first 30 days matter.

Days 1 to 7

• Read the letter carefully and identify which type it is
• Note any deadline HMRC has set for a response
• Do not ignore it or delay -- even informal letters can escalate
• Contact your accountant or tax adviser immediately if you use one

Days 7 to 14

• Gather all records relating to the tax year(s) mentioned:
  • Bank statements (personal and business)
  • Invoices and receipts
  • PAYE records and payslips
  • Property purchase, sale and rental records
  • Investment and share dealing records
  • Any offshore account statements

Days 14 to 30

• Review your submitted return against your records
• Identify any discrepancies -- errors in your favour or HMRC's
• Decide whether you need professional representation
• Draft or instruct your adviser to draft an initial response

Records You Should Gather

HMRC can request records going back six years for careless errors (20 years for fraud). Do not destroy anything during an active enquiry. Key records include:

The Penalty Framework

HMRC applies penalties based on the taxpayer's behaviour -- not merely the amount of tax underpaid.

"Unprompted" means you disclosed before HMRC raised the issue. "Prompted" means you disclosed after HMRC contacted you. Both are better than HMRC discovering the issue itself, which leads to maximum penalties.

Penalties are also reduced for the quality of disclosure -- telling HMRC what happened, helping them understand the position and giving them access to records all reduce the final penalty.

When to Get Professional Help

Always seek professional help if:

• The letter is a COP9 -- this is non-negotiable
• The amounts involved are above GBP 10,000
• The enquiry covers multiple tax years
• You have offshore assets or income
• You are unsure whether your original return was correct
• HMRC's information request is broad or unclear

The cost of professional representation is almost always less than the cost of a poorly handled enquiry -- both in additional tax and in unnecessary penalties.

Making a Voluntary Disclosure

If you identify an error before HMRC does -- whether triggered by a nudge letter or your own review -- making an unprompted voluntary disclosure via HMRC's Digital Disclosure Service can reduce penalties to zero for careless errors. The process requires you to:

1. Notify HMRC of your intention to disclose
2. Calculate the additional tax, interest and penalties owed
3. Submit a complete disclosure and pay in full

Late payment interest currently runs at 7.25% per annum on unpaid tax, accruing from the original due date. Prompt payment after disclosure limits the interest charge.

Use the CalcHub HMRC penalty and interest calculator to estimate the total cost of a voluntary disclosure -- including late payment interest -- before you submit, so you know exactly what to pay and can avoid underpaying accidentally.